Image guidance in radiotherapy is an active area of commercial development and research. Systems using megavolt (MV) detectors, kilovolt (kV) X-ray sources and detectors, ultrasound, and radiofrequency emitting fiducials have been described and commercialized. To this end, there has been interest in combining a linear accelerator, used in radiotherapy, with magnetic resonance imaging (MRI) to improve image guidance. MRI offers relatively good soft tissue contrast and can image in any arbitrary plane in the patient, including the beam's-eye view, which may be useful in radiotherapy. The first demonstration of this principle was first described by Hammer and colleagues1, and more recently two other groups have also demonstrated that a linear accelerator and MRI can co-exist in even closer proximity2,3.
Some difficulties with integrating MRI with radiotherapy may include the following:
1. The linear accelerator operates by accelerating an electron to high energies before converting this energy to X-rays by smashing the electron into a target and the bremsstrahlung process. The motion of the electron in the linear accelerator is thus affected by a magnetic field, such as that required for MRI, by the Lorentz force, F=ev×B (where F is the force exerted on a particle, e is the charge of the particle, v is the instantaneous velocity of the particle and B is the strength of the magnetic field), which could prevent or hamper the functionality of the linear accelerator.
2. MRI typically require a relatively high degree of homogeneity in the magnetic field for imaging, which is most easily achieved by constructing the MRI magnet such that it surrounds the patient, and may thus prevent or impede open access to the patient by the linear accelerator.
3. The radiotherapy dose distribution in the patient is affected by the magnetic field in the patient since the electrons scattered by the incident photons are also affected by the magnetic field of the MRI by the Lorentz force F=ev×B4-15. This disturbance of the dose distribution can be significant, particularly at higher magnetic fields, and can yield considerable dose differences, especially in regions in the patient where there is a tissue density inhomogeneity. In this case an effect called the “dose return effect9” has been described where scattered electrons in a distal lower density medium have a sufficiently large range such that their trajectory can return to a proximal higher density tissue, and deposit an unintended supplemental dose.
4. Other difficulties also exist such as radiofrequency (RF) interference16-19, however it has been demonstrated that these can effectively be removed or diminished by shielding2,3.
To overcome the first three problems, several designs of an integrated MRI and linear accelerator have been proposed, and several patents and patent applications have been published20-25. With the exception of Amies et al20 and Carlone et al26, these solutions seek to avoid the first problem listed above by removing the magnetic field from the MRI in the area where the linear accelerator will be located. This may be done by passive or active magnetic shielding of the linear accelerator. This approach may have some difficulties. One difficulty is that the introduction of shielding affects the design of the MRI magnet; the complexity of a magnet that can shield a region for a linear accelerator and still maintain parts-per-million (ppm) homogeneity at the imaging zone is typically higher than typical MRI magnets. Another difficulty is that this method encourages the use of distance to facilitate magnetic isolation, which may be a drawback since a compact design may be useful to facilitate the use of this technology in a hospital setting, where space is often restricted. Another difficulty is that this method may push the fringe field of the MRI magnet outward since a low field zone is generated near the centre of the MRI. This extension of the fringe field can be a serious problem, and can affect the MRI facility specification.
The second and third problems listed above, generally, are not completely resolved by the conventional or suggested methods described above. The conventional approaches used to overcome the second problem (i.e., need for direct view of the patient within the MRI by the linear accelerator) include:
1. To use an open magnet configuration and rotate the magnet about the patient2,22,23;
2. To employ a cylindrical MRI magnet that has been separated by a small gap, and irradiate through the gap25;
3. To use an MRI magnet specifically designed for interventional radiology21,27; or
4. To use an MRI magnet that is large enough to accept the entire linear accelerator within its bore20.
None of the conventional or proposed linear accelerator designs described above overcomes the third problem (i.e., perturbation of the dose distribution in the patient by the magnetic field of the MRI), but some methods have been suggested to minimize this effect, including:
1. To use a low magnetic field such that the effect is small5, or
2. To employ complex beam geometries to compensate for the perturbed dose distribution8.
It should be noted that conventional and suggested solutions for MRI guidance of radiotherapy typically does not consider that isolation of the linear accelerator from a magnetic field is not required to maintain functionality of the linear accelerator. Focusing of charged particles by electromagnetic forces is a field of investigation that has been used in areas of ion implantation, ion gun design, high energy particle accelerators, and many others28,29. As well, magnetic field focusing of electrons in traveling wave linear accelerators is a component of their design, and these devices would not function without such focusing30,31. The principle of such focusing is that the linear accelerator is immersed in a non-uniform magnetic field that is parallel to the direction of motion of the particle travel. Particles that are precisely on the central axis of the magnetic field have no magnetic force exerted upon them, and so they stay aligned with the central axis. Particles that are parallel to the magnetic field, but off-axis (because they were miss-aligned when injected, for instance), will interact with the radial component of the magnetic field causing a circular trajectory about the central axis of the magnetic field, and perpendicular to this axis. Such motion results in a central and focusing force upon the charged particle because of interaction with the axial magnetic field (for example, a complete description is given on p. 126 of Humphreys30).
Based on this observation, it may be possible to integrate a linear accelerator into an MRI magnet and maintain functionality of the linear accelerator if its orientation were such that it was parallel to the magnetic field of the MRI, and pointed towards the MRI's isocenter. One way of doing this may be to use a parallel coil configuration, such as the ones used in commercially available open magnets, with the linear accelerator placed in the centre of one of the coils, for example as described in the patent application by Carlone et al26. An example of this method is illustrated in FIG. 1.
FIG. 1 shows an example magnet coil 1 and an example linear accelerator 2. In this example the current flow in the magnet coil is shown to be clockwise 3 and produces a magnetic field direction that is downward 5. The electronic motion inside the linear accelerator is downward 4 and parallel to the direction of the magnetic field 5 and so the linear accelerator may function well while in the magnetic field and may not need to be isolated from it. The imaging target 6 may be positioned inside the X-ray field 7, whose direction is also parallel to the direction of the MRI magnetic field.
This arrangement may address problems 1 and 3 listed in the above. Problem one may be overcome since the method does not rely on magnetic shielding for linear accelerator integration into an MRI magnet. Problem 3 may be also overcome because, as described by Bielajew4, the scattered electrons generated by photon interactions are also focused by the magnetic field, which result in less lateral scatter. The undesirable “electron return effect” as described by Raaijmakers and colleagues9 may be eliminated for this configuration. However, other problems may arise, as will be described below.